1. Field of the Invention
invention relates to an alkaline storage battery such as a sealed nickel-cadmium storage battery or the like, and more particularly to an alkaline storage battery having an improved current collector tab structure for increased high-rate discharge performance.
2. Prior Art
One conventional alkaline storage battery is shown in FIGS. 8 and 9 of the accompanying drawings. The known alkaline storage battery has an electrode assembly 34 comprising a positive electrode 32 and a negative electrode 33 which are wound as spiral coils with a separator 31 interposed therebetween. The electrode assembly 34 is housed in a nickel-plated steel case 35 which doubles as the negative terminal of the storage battery. The positive electrode 32 has a terminal edge 36 projecting from one end of the electrode assembly 34, whereas the negative electrode 33 has a terminal edge 37 projecting from the other end of the electrode assembly 34. The terminal edge 36 is connected to an upper current collector tab 38 which comprises a circular collecting tab 39 welded to the terminal edge 36 and a rectangular lead tab 40 extending from the circular collecting tab 39, the rectangular lead tab 40 having a width smaller than the diameter of the circular collecting tab 39. The lead tab 40 is spot-welded to a sealing cover 41 of the case 35, the sealing cover 41 serving as the positive terminal of the storage battery. The terminal edge 37 is connected to a lower current collector tab 42 which comprises a circular collecting tab 43 welded to the terminal edge 37 and a tongue-shaped lead tab 44 attached to the center of the circular collecting tab 43. The lead tab 44 is spot-welded to the inner surface of the bottom of the case 35 at the center of the circular collecting tab 43.
It is known that the high-rate discharge performance of the alkaline storage battery of such a construction greatly varies depending on the internal resistance of the storage battery.
The current collector tabs 38, 42 are spot-welded to the terminal edges 36, 37, respectively, of the electrode assembly 34. To spot-weld such a current collector tab to a terminal edge of an electrode assembly, one surface of the current collector tab is held against the terminal edge of the electrode assembly, and a pair of spot-welding electrodes is disposed on the other surface of the current collector tab. The welding current which is supplied between the spot-welding electrodes flows from the terminal edge of the electrode assembly to the electrode assembly itself through the current collector tab. If the current collector tab, with its current collecting tab and lead tab having the same thickness, is increased in its overall thickness to reduce the electric resistance thereof, then when the current collector tab is welded to the terminal edge of the electrode assembly, the loss current flowing through the current collector tab is increased, thereby reducing the welding current flowing through the joint between the terminal edge of the electrode assembly and the current collector tab. As a result, the joint cannot be welded effectively. It is therefore preferable to reduce the thickness of the current collector tab in order to minimize the loss current which flows during the welding process. If the current collector tab is thinned, however, the electric resistance of the lead tab on which the current is concentrated is increased. Consequently, it is impossible to reduce the electric resistance of the lead tab and hence the entire current collector tab.